'70 Mustang tach dash conversion question

I found a 1970 Cougar carcass with a tach dash. I took the tach and
have jerry-rigged it to my '70 Mustang's instrument cluster. Looks
good with the white face overlay. I've grounded the back of the tach
to the car's chassis and have run one of the two leads to my ignition
switch for power and the other to the negative side of my coil. The
engine fires up and the tach registers but the car runs extremely
rough. As soon as I disconnect the wire from the coil, she runs
smoothly again. I seem to remember reading something about this
(perhaps about an 8K ohm resistor?) in one of my Mustang Monthlies but
which one? Does anyone have the scoop on this? If so, what part do I
ask for when I go to UAP, Canadian Tire, Radio Shack or wherever?
Please give me as much detail as you can because, a mechanic or an
electrician I definitely am not.

Thanks in advance,
Richard

On Mar 14, 10:00 pm, "lxixboss" > wrote:
> I found a 1970 Cougar carcass with a tach dash. I took the tach and
> have jerry-rigged it to my '70 Mustang's instrument cluster. Looks
> good with the white face overlay. I've grounded the back of the tach
> to the car's chassis and have run one of the two leads to my ignition
> switch for power and the other to the negative side of my coil. The
> engine fires up and the tach registers but the car runs extremely
> rough. As soon as I disconnect the wire from the coil, she runs
> smoothly again. I seem to remember reading something about this
> (perhaps about an 8K ohm resistor?) in one of my Mustang Monthlies but
> which one? Does anyone have the scoop on this? If so, what part do I
> ask for when I go to UAP, Canadian Tire, Radio Shack or wherever?
> Please give me as much detail as you can because, a mechanic or an
> electrician I definitely am not.
>
> Thanks in advance,
> Richard

OK. Got it fixed, All I had to do was to splice both tach wires into
the "big, soft, fat, pink" wire coming out of the fuse box.

Richard